The present invention relates to geopolymer composite binders for cement and concrete and methods of making and using thereof.
Geopolymers comprise of silicon and aluminum atoms bonded via oxygen atoms into a polymer network. Geopolymers are prepared by dissolution and poly-condensation reactions between a reactive aluminosilicate material and an alkaline silicate solution, such as a mixture of an alkali metal silicate and metal hydroxide. Examples of a reactive aluminosilicate material are Class F fly ash (FFA) and Class C fly ash (CFA).
Fly ash is a fine powder byproduct formed from the combustion of coal. Electric power plant utility furnaces burning pulverized coal produce most of the commercially available fly ashes. These fly ashes comprise mainly of glassy spherical particles, as well as hematite and magnetite, unburned carbon, and some crystalline phases formed during cooling. The structure, composition and properties of fly ash particles depend upon the composition of the coal and the combustion process by which fly ash is formed. American Society for Testing and Materials (ASTM) C618 standard recognizes two major classes of fly ashes for use in concrete: Class C and Class F. All ASTM standards and their specifications described in this disclosure are incorporated by reference in their entirety. Class F fly ash is normally produced from burning anthracite or bituminous coal, whereas Class C fly ash is normally produced from lignite or sub-bituminous coal. The ASTM C618 standard differentiates Class F and Class C fly ashes primarily according to their pozzolanic properties. Accordingly, in the ASTM C618 standard, one major specification difference between the Class F fly ash and Class C fly ash is the lower limit of (SiO2+Al2O3+Fe2O3) in the composition. The lower limit of (SiO2+Al2O3Fe2O3) for Class F fly ash is 70% and that for Class C fly ash it is 50%. Accordingly, Class F fly ashes generally have a calcium oxide content of about 15 wt % or less, whereas Class C fly ashes generally have a higher calcium oxide content (e.g., higher than 15 wt %, such as 20 to 40 wt %). A high calcium oxide content makes Class C fly ashes possess cementitious properties leading to the formation of calcium silicate and calcium aluminate hydrates when mixed with water.
Depending on the chemical composition and method of production, ground granulated blast furnace slag (GGBFS) is a glassy granular material that varies, from a coarse, popcorn-like friable structure greater than 4.75 mm in diameter to dense, sand-size grains. Grinding reduces the particle size to cement fineness, allowing its use as a supplementary cementitious material in Portland cement-based concrete. Typical ground granulated blast furnace slag includes 27-38% SiO2, 7-12% Al2O3, 34-43% CaO, 7-15% MgO, 0.2-1.6% Fe2O3, 0.15-0.76% MnO and 1.0-1.9% by weight. Since GGBFS is almost 100% glassy (or “amorphous”), it is generally more reactive than most fly ashes. GGBFS produces a higher proportion of the strength-enhancing calcium silicate hydrate (CSH) than Portland cement, thereby resulting in higher ultimate strength than concrete made with Portland cement.
In contrast to the concrete formed from Ordinary Portland Cement (OPC), a geopolymer concrete can exhibit greater heat-, fire- and acid-resistance. The process of forming geopolymers involves a dissolution/condensation/poly-condensation/polymerization reaction, which begins as soon as certain aluminosilicate materials are exposed to an alkaline solution.
One challenge is that Low Ca Class F fly ash based geopolymer concrete hardens very slowly and has a low final strength, particularly if cured at low temperatures (e.g., room temperature). This finding is consistent with observations in the literature. On the other hand, an increase in the Ca content can decrease the setting time, which can sometimes result in cracks in the products.
In addition, the setting time for a Class F fly ash based geopolymer decreases with increasing CaO content. For example, a Class F fly ash with about 12 wt % CaO sets in less than 40 minutes. As a consequence, micro-cracking occurs due to shrinkage, resulting in a low strength when samples are cured at room temperature.
Thus, a need exists to overcome these challenges to have a geopolymer that can maintain its final strength even when it is cured at a low temperature and at the same time has a sufficiently long setting time to mitigate micro-cracking.